Comparators are commonly employed for threshold detection applications, where the output of the comparator changes state depending on whether a variable input voltage is above or below a reference voltage. For example, a comparator can be used as a zero-crossing detector to provide a logic level digital output signal based on an AC input voltage developed by a magnetic variable reluctance speed sensor.
In applications where electrical noise is superimposed on an input signal, the comparator may change states based on the noise content of the input signal when the input voltage approaches the switch point of the comparator. To desensitize the comparator to such noise, the comparator circuit is designed with a hysteresis characteristic, which effectively increases or decreases the reference voltage, depending on the output state of the comparator.
In the state of the art, various different implementations of hysteresis comparators are known. A conventional technique makes use of an operational amplifier and resistive feedback to provide the hysteresis characteristic. U.S. Pat. No. 5,369,319 A for instance describes a MOS hysteresis comparator, which is defined by a current source transistor Qs feeding a differential transistor pair Q5, Q6, each of which is connected in series with a respective load transistor Q1, Q4. Each series connection of a differential transistor Q5 or Q6 and a load transistor Q1 or Q4 forms a respective first or second comparator leg. Hysteresis transistors Q2 and Q3 are cross-coupled between the first and second comparator legs and efficiently shift the switching point of the comparator to achieve a hysteresis characteristic. U.S. Pat. No. 5,369,319 further describes a source transistor bias circuit to compensate for process, voltage and temperature (PVT) variations.
Although U.S. Pat. No. 5,369,319 suggests a source transistor bias circuit, which enables the compensation of PVT variations of the hysteresis voltage, the proposed source transistor bias circuit has several drawbacks:
increased area and power requirement because of the need of an error amplifier;
risk of instability because of the need of a feedback loop; and
a start-up signal and circuitry because of an initial undefined state after putting proposed current source transistor bias circuit into operation.
Hence, there is still a need for a metal oxide semiconductor (MOS) hysteresis comparator circuit, and more particularly, a MOS hysteresis comparator circuit having a hysteresis characteristic, which is substantially unaffected by variations in temperature and manufacturing process.